Corrosion inhibition with molecularly dehydrated phosphate glass



3,067,024 CORROSHGN I. ITEON Wl'lH MOLECULARLY DEHYDRATED PHOSPHATE GLASS David B. Boies, Chicago, Jacob I. Bregman, Park Forest, and Theodore R. Newman, Dal; Lawn, EL, assignors to Nalco Chemicai Company, a corporation of Delaware No Drawing. Filed Sept. 13, 1957, Ser. No. 683,716 4 Claims. (Cl. 71-50) This invention relates to corrosion inhibitors for preventing the corrosion of tanks and equipment used in the manufacture, storage, transportation and use of concentrated solutions of ammonium nitrate and ammonia-ammonium nitrate mixtures.

The production and use of nitrogen fertilizer solutions is becoming increasingly popular. Distributing these corrosive solutions, however, is a real problem. Ammonium nitrate solutions and ammonium nitrate solutions containing ammonia gas dissolved therewith eat through standard carbon steel containers in about three years. Present protective devices for the steel containers such as coatings add little to this life span. Small cracks or pin holes in plastic coatings have proved to be fatal to the coating. Once the nitrogen solution works through to the underlying metal, it corrodes the metal container, and the coating begins to fiake ofi. Rubber coatings, on the other hand, are reported to be vulnerable at the seams. Glass linings and stainless steel cladding are too expensive to provide an economical answer to the corrosion problem. In the light of the number of standard carbon steel containers involved in the distribution of nitrogen fertilizer solutions (tank cars, distributor storage tanks, tank trucks, farmers storage tanks, field tanks and applicator tanks), this corrosion problem assumes sizable proportions.

Several attempts have been made to solve this problem by using chemical inhibitors but for the most part they rave proved unsatisfactory since they give only partial protection and the amounts required to perform satisfactorily are often excessive. To be effective, an inhibitor must not only work at low dosages but must protect metal surfaces in direct contact with these solutions and also those surfaces exposed to the interface and the atmosphere directly surrounding these solutions.

The most common use of concentrated solutions of ammonium nitrate is in the production of the well known mixed goods type fertilizers from super phosphates. These solutions, as previously indicated, usually also contain ammonia. Several typical ammonia-ammonium nitrate solutions in terms of their compositions and properties are listed below in Table I.

TABLE I I II III Total N, percent 40. G 40. 8 37. Anhydrous NHa, percent 21. 7 26 0 16. 0 NOi-N, percent 11.37 9. 65 11. 68 Total NH3N, percent 29. 24 81. 8 25. 35 Water, percent 13. 3 18. 5 16. 6 Specific Gravity at 60 1. 142 1.097 1.182 Vapor pressure in p.s.i. at 104 F 10 16 1 3,%7,24 Patented Dec. 4, 1.962

A further object is to prevent the corrosion of metals, particularly ferrous metals, caused by contact with corrosive solutions of ammonium nitrate and ammonia-ammonium nitrate mixtures. Other objects will appear hereinafter.

in accordance with the invention, it has been found that the corrosiveness of ammonium nitrate and ammoniaarnmonium nitrate solutions may be prevented or substantially reduced by using minor, yet corrosion-inhibiting, amounts of water soluble molecularly dehydrated phosphates. The amount necessary to perform satisfactorily may be as little as parts per million (p.p.m.) but usually the dosage will range from 250 to 1,000 ppm. with greater amounts being excessive since the improvements obtained are inconsequential. While any of the alkali metal salts of the molecularly dehydrated phosphates may be used, the sodium salt is preferred since it gives good results and is the most readily available commercial form.

Examples of several molecularly dehydrated phosphates that may be used are: sodium tripolyphosphate, Na P O tetra-sodium pyrophosphate, Na P o sodium decaphosphate, Na P O and sodium hexametaphosphate, Na P O It will be understood that any alkali metal phosphate having an alkali metal oxide, expressed as Na O, to P 0 molecular ratio of 1:1 to 2:1 may be used.

In a preferred embodiment it is beneficial to use a molecularly dehydrated phosphate glass such as sodium decaphosphate (63.5% P 0 or sodium hexametaphos phate (67% P 0 or other similar phosphate glass having an Na O to P 0 molecular ratio within the range of 1:1 to 1.34:1. Also preferred are the alkali metal tripolyphosphates.

Another important embodiment of the invention resides in the use of molecularly dehydrated phosphates in combination with alkali metal carbonates or with water soluble salts of orthophosphoric acid. When used in combination the weight ratio of molecularly dehydrated phosphate to' alkali metal carbonate or orthophosphate is within the range of 1:2 to 1:20 with the preferred range being at about 1:4 to 1:8. To achieve optimum effects the amount of molecularly dehydrated phosphate used should be at least 100 ppm. e

In order to demonstrate the types of compositions used in developing the invention, the following are presented:

Composition I Percent Sodium decaphosphate; Na P O P 0 ..t 63.5

1 Composition 11 f Sodium tripolyphosphate; Na P O P 0 57 1 Composition 111 Sodium hexametaphosphate; Na P O P 0 67 Composition IV Sodium carbonate.

Composition V Composition VII Sodium dichromate.

Composition VIII Ammonium thiocyanate.

Composition IX Sodium metasilicate.

3 Composition X Ingredients: Percent by weight 1- (Z-hydroxyethyl) -1-benzyl-2-heptadecyl imidazolinum nitrite 20 Water-isopropanol (solvent) 8O Composition XI Percent Trisodium orthophosphate; Na POflas P 38.5

EXAMPLE To evaluate the invention the following test method was employed:

A one-pint bottle was used with 300 ml. of the test solution. The specimens were held by means of glass hooks, one completely submerged and the other one-half submerged. The coupons were immersed for 18 hours at room temperature (72 F.i5; 1925 (3.), at the end of which time they were removed, cleaned, re-weighed and the corrosion rate calculated in m.p.y. (mils per year) and in terms of percent protection, i.e. percent reduction in the corrosion rate as compared to a blank. The interface coupon was observed for increased corrosion or pitting at the air-solution interface. The bottles were vented to the atmosphere.

Mild steel sandblasted coupons, inch x 1 /2 inch x 16 gauge were the test specimens.

The test medium was prepared by dissolving 60 parts of commercial ammonium nitrate in 40 parts of water.

The results of the corrosion tests in 60% ammonium nitrate solutions are presented in Table II. The results are given in mils per year (m.p.y.) for both specimens, base on exposed area, and also in terms of percent protection, based on average blank weight losses of 127 m.p.y. for the submerged specimen, and 163 m.p.y. for the inter face specimen. These figures are the average of nine blank tests. The range of corrosion rates for these blank tests was 117 to 138 m.p.y. for the submerged coupon, and 135 to 193 m.p.y. for the interface coupon.

TABLE II Corrosion Rate Percent Protection 92 52 15 1? 3 1% Sub Inter Sub Inter merged, face, merged face m.p.y. m.p.y.

1--- Blank.-. 127 1g3 3 3 i 0 6 5 5 100(3) 44 65 65 60 2 I 250E343 21 29 11 1, 000 (2) 4 5 97 97 0(2) 53 114 59 30 H a a 2 r a 50 4 5 500(1) 15 24 88 85 50 (1) 1 86 98 32 39 $2828 53 it 9% 9% 500(1) 25 26 80 84 50(1) 140 172 O 0 5 VII OUE2; 122 145 g I;

50 2 1 4 154 600(1) 67 108 47 33 100(1) 122 156 4 4 6 VIII 135 155 0 5 0 0 181 0 0 1, 000(2) 130 219 5 0 7 1, 5000;; 15181 14% 92 113 8 1 8 0 7 9 X 500(1) .152 107 0 33 100(1) 101 152 21 7 250(1) 58 7O 53 57 l0 V 500 (7) b0 29 61 82 750 (4) 13 6 90 96 1, 200 E1; 4 9 97 94 11 32 91 so 13 23 so s2 s2 14--. 250(1) 12 14 90 91 *Numbers in parentheses indicatethenumberof tests run at a given concentration.

Table II clearly demonstrates the several advantages of the invention. Substantial protection for both the submerged and the partially submerged specimens were obtained with as little as 250 p.p.m. of molecularly dehydrated phosphates (Tests Nos. 2-4).

Test N o. 5 demonstrates that the well known inhibitor, sodium dichromate, is ineffectual at dosages as high as 500 ppm. The inhibitor, ammonium thiocyanate, which is used cornmeircially to inhibit solutions of the type under test, gave little or no protection at dosages as high as 1,000 parts per million (Test No. 6). In a similar manner, Compositions IX and X, which are well known commercial inhibitors, were relatively ineifective at 500 parts per million (Tests Nos. 8 and 9).

When tested alone at 1,500 p.p.m., sodium carbonate gave good interface protection but failed to protect the submerged specimen (Test No. 7). Test No. 10 demonstrates the advantages of combining alkali metal carbonates with molecularly dehydrated phosphates. This point is also demonstrated in Tests Nos. 11 and 12.

Tests 13 and 14 illustrate the benefits derived when a molecularly dehydrated phosphate is combined with an alkali metal orthophosphate.

The invention provides a simple and effective means of preventing the corrosive effects of solutions containing ammonium nitrate, especially solutions containing ammonium nitrate and water in which the weight ratio of ammonium nitrate to ammonium nitrate plus water is 40% to 75%. The inhibitors of the invention are cheap, readily available and work at low, economical dosages. It is also interesting to note that the inhibitors of the invention, when used at recommended concentrations, do not affect the crystallization temperatures of the fertilizer solutions.

The invention is hereby claimed as follows:

1. A non-corrosive liquid fertilizer comprising water containing a major portion of a fertilizer from the group consisting of ammonium nitrate and ammonia-ammonium nitrate mixtures and from 100 to 1,000 parts per million of a molecularly dehydrated phosphate glass having the empirical formula Na P O and a P 0 content of about 63.5% by weight.

2. A non-corrosive liquid fertilizer comprising water containing a major portion of a fertilizer from the group consisting of ammonium nitrate and ammonia-ammonium nitrate mixtures and from 100 to 1,000 parts per million of sodium tripolyphosphate.

3. A non-corrosive liquid fertilizer comprising water containing a major portion of a fertilizer from the group consisting of ammonium nitrate and ammonia-ammonium nitrate mixtures and a composition comprising (A) a molecularly dehydrated phosphate glass having the empirical formula Na P O and a P 0 content of about 63.5% by weight and (B) sodium carbonate, with the weight ratio of A to B being. within the range of 1:4 to 1:8, and with the proviso that the total phosphate content of said liquidfertilizer is at least parts per million.

4. A non-corrosive liquid fertilizer comprising water containing a major portion of a fertilizer from the group consisting of ammonium nitrate and ammonia-ammonium nitrate mixtures and a composition comprising (A) sodium tripolyphosphate and (B) sodium carbonate, with the weight ratio of A to B being within the range of 1:4 to 1:8, and with the proviso that the total phosphate content of said liquid fertilizer is at least 100 parts per million. 7

References Cited in the file of this patent UNITED STATES PATENTS 2,337,856 Rice et al. Dec. 28, 1943 2,715,059 Miller Aug. 9, 1955 2,739,886 Facer Mar. 27, 1956 2,770,538 Vierling Nov. 13, 1956 2,781,254 Munekata et al Feb. 12, 1957 2,855,286 Harvey Oct. 7, 1958 

1. A NON-CORROSIVE LIQUID FERTILIZER COMPRISING WATER CONTAINING A MAJOR PORTION OF A FERTILIZER FROM THE GROUP CONSISTING OF AMMONIUM NITRATE AND AMMONIA-AMMONIUM NITRATE MIXTURES AND FROM 100 TO 1,000 PARTS PER MILLION OF A MOLECULARLY DEHYDRATED PHOSPHATE GLASS HAVING THE EMPIRICAL FORMULA NA12P10O31 AND A P2O5 CONTENT OF ABOUT 63.5% BY WEIGHT. 